分布式锁

 1 public class ZKLockDemo {
    public static void main(String[] args) throws Exception {

        String zkConnection = "192.168.40.136:2181,192.168.40.137:2181,192.168.40.138:2181";
      
        //重试策略 maxRetries：最大重试次数 baseSleepTimeMs：初始sleep时间
        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
        //采用Fluent风格初始会话
        CuratorFramework client = CuratorFrameworkFactory.builder().connectString(zkConnection).sessionTimeoutMs(5000).retryPolicy(retryPolicy).build();
        //启动会话
        client.start();

        //创建分布式锁, 锁空间的根节点路径为/lock
        InterProcessMutex mutex = new InterProcessMutex(client, "/lock");
        //获取锁
        mutex.acquire();

        //获得了锁, 进行业务流程
        System.out.println("Enter mutex");

        //完成业务流程, 释放锁
        mutex.release();
        //关闭客户端
        client.close();

    }
}

获取锁代码：

@Override
    public void acquire() throws Exception
    {
        if ( !internalLock(-1, null) )
        {
            throw new IOException("Lost connection while trying to acquire lock: " + basePath);
        }
    }

@Override
    public boolean acquire(long time, TimeUnit unit) throws Exception
    {
        return internalLock(time, unit);
    }

internalLock(time, unit)中，time=-1表示锁被占用时，永久阻塞

private boolean internalLock(long time, TimeUnit unit) throws Exception
    {
        /*
           Note on concurrency: a given lockData instance
           can be only acted on by a single thread so locking isn't necessary
        */

        Thread currentThread = Thread.currentThread();
        
        LockData lockData = threadData.get(currentThread);
        if ( lockData != null )
        {
            // 可重入锁，计数器加一，在释放时需要释放多次
            lockData.lockCount.incrementAndGet();
            return true;
        }
        //获取锁
        String lockPath = internals.attemptLock(time, unit, getLockNodeBytes());
        if ( lockPath != null )
        {
            LockData newLockData = new LockData(currentThread, lockPath);
            threadData.put(currentThread, newLockData);
            return true;
        }

        return false;
    }    

String attemptLock(long time, TimeUnit unit, byte[] lockNodeBytes) throws Exception
    {
        final long      startMillis = System.currentTimeMillis();
        final Long      millisToWait = (unit != null) ? unit.toMillis(time) : null;
        final byte[]    localLockNodeBytes = (revocable.get() != null) ? new byte[0] : lockNodeBytes;
        int             retryCount = 0;

        String          ourPath = null;
        boolean         hasTheLock = false;
        boolean         isDone = false;
        //自选获取锁
        while ( !isDone )
        {
            isDone = true;

            try
            {
                //创建临时有序子节点
                ourPath = driver.createsTheLock(client, path, localLockNodeBytes);
                ////判断是否获得锁（子节点序号最小），获得锁则直接返回，否则阻塞等待前一个子节点删除的通知
                hasTheLock = internalLockLoop(startMillis, millisToWait, ourPath);
            }
            catch ( KeeperException.NoNodeException e )
            {
                // gets thrown by StandardLockInternalsDriver when it can't find the lock node
                // this can happen when the session expires, etc. So, if the retry allows, just try it all again
                if ( client.getZookeeperClient().getRetryPolicy().allowRetry(retryCount++, 
                       System.currentTimeMillis() - startMillis, RetryLoop.getDefaultRetrySleeper()) )
                {
                    isDone = false;
                }
                else
                {
                    throw e;
                }
            }
        }

        if ( hasTheLock )
        {
            return ourPath;
        }

        return null;
    }    

优点：

1.  可靠性高、实现简单

2.  zookeeper因为临时节点的特性，如果因为其他客户端因为异常和zookeeper连接中断了，那么节点会被删除，意味着锁会被自动释放

3.  zookeeper本身提供了一套很好的集群方案，比较稳定

4.  释放锁操作，会有watch通知机制，也就是服务器端会主动发送消息给客户端这个锁已经被释放了

数据库实现分布式锁：

建表语句：

CREATE TABLE `lock` (
    `id` INT NOT NULL AUTO_INCREMENT,
    `method_name` VARCHAR(200) NOT NULL DEFAULT '0',
    `create_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    `update_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    PRIMARY KEY (`id`),
    UNIQUE INDEX `method_name` (`method_name`)
)
COMMENT='分布式锁'
COLLATE='utf8mb4_0900_ai_ci'
;

获取锁伪代码：

try{
    insert into lock(method_name) values(‘methodName’);   
    return true;
}Catch(DuplicateException e){
    return false;
}

释放锁：

delete from lock where method_name="methodName";

缺点：

1.   锁没有失效时间，一旦解锁操作失败，就会导致锁记录一直在数据库中，其他线程无法再获得到锁

2.   锁是非阻塞的，数据的insert操作，一旦插入失败就会直接报错。没有获得锁的线程并不会进入排队队列，要想再次获得锁就要再次触发获得锁操作

3.   锁是非重入的，同一个线程在没有释放锁之前无法再次获得该

基于缓存的分布式锁实现

以后补充

参考：

https://blog.csdn.net/qiangcuo6087/article/details/79067136